The effects of ultrasonic power, duration and treatment temperature on molten Mg-3Ca alloy degassing and solidification structure were investigated. The results indicate that applying of ultrasonic treatment technology can effectively remove gas and increase the density of Mg-3Ca alloy. Ultrasonic degassing effect is closely related with ultrasonic power, duration and treatment temperature. Too low ultrasonic power, too short or too long ultrasonic duration and too high or too low temperature are not beneficial to degassing effect. Under this experiment condition, when ultrasonic power is 150 W, ultrasonic duration is 120 s and treatment temperature is 700 ℃, the optimum degassing effect can be obtained and the degassing efficiency reaches 53.8%. In addition, ultrasound not only removes gas in melt, but also obtains the uniform refining solidification structure of ingot at the same time. Coarsened dendrite is adverse to bubbles rising and melt feeding. However, uniform refining equiaxed grain is beneficial to bubbles rising and melts feeding.